U.S. Pat. No. 7,178,353 and U.S. Pat. No. 7,415,835 disclose a novel temperature control concept which has been termed Transfer Direct of Saturated Fluids (TDSF) and which concept has material advantages for many temperature control systems used in modem high technology processes. In accordance with this concept, a two-phase fluid that is being used for temperature control is first compressed to a high pressure, high temperature gas, which is variably divided into two flows, the mass flow of one of which is directly controlled. The remaining second flow is condensed to a liquid state by cooling and then cooled further by being expanded to a vapor or liquid/vapor state as a saturated mist. The flow of hot compressed gas is varied under command signals derived from sensing the load temperature to a level such that the pressurized component, when mixed with the differential flow of saturated liquid/vapor mist provides a combined output of selectively controlled enthalpy, pressure and temperature. The load temperature is thus adjusted to the desired target level by direct control of the proportion of gas flow alone, accompanied by concurrent indirect control of the second or liquid/vapor state. After thermal exchange of the combined flow with the thermal load, the two-phase fluid is returned, using appropriate conditioning, for recompression in a closed loop operation.
Systems and method employing two-phase refrigerant in direct contact with a thermal load in this manner can provide precise but changeable control of load temperature over a wide dynamic range. The concept of uniting a variably pressurized two-phase refrigerant with a differential flow of saturated mist component to provide precise and stable temperature control of a thermal load has many advantages. The thermodynamic cycle is efficient, and also avoids the costs and thermal losses involved in using a separate thermal exchange medium. Additionally, because the process relies on pressure as well as enthalpy, rapid changes in a precisely defined target temperature can sometimes be introduced by pressure change alone. Where the temperature to be controlled and the heat load permit, temperature stabilization can be aided by use of the latent heat of evaporation or condensation in a two-phase mixture at thermal equilibrium.
The TDSF concept, therefore, not only has many current applications but also potential for more and different embodiments. Some operative non-uniformities which can be encountered, for example in some semiconductor manufacturing operations, have given rise to novel problems. When processing a semiconductor wafer, for example, the wafer may be mounted with its base side in contact with a thermally controlled (cooled) support platen. Then the upper side of the wafer, typically after having received a patterned protective layer, is exposed to a very high intensity energy source, such as a plasma, and a desired pattern is etched by bombardment into the wafer surface.
The TDSF system has proven to be superior for these, as well as other applications in general. Because of the historical trend in the semiconductor industry toward use of wafers of ever larger size, (wafers may now be greater than 300 mm diameter) the problems involved in laying down essentially microscopic high density patterns uniformly across the wafer area have been exacerbated. Moreover, significant differences in thermal exchange characteristics can exist between different areas of such large wafer surfaces. For example, the intensity of the heating source may vary materially across different areas of the wafer and thermal exchange efficiency may also vary somewhat with location on the wafer. Such variations can unacceptably reduce output yield in terms of the percentage of high quality micro images that can be formed within the patterns introduced into the wafer surface.
Even though a thermal control system based upon the TDSF concept provides a stable and precise temperature source at a target level, areal dispersion of the flowing plasma or other medium across the wafer may thus not be uniform. Variations in heat loads and heat transfer across different areas may therefore have to be compensated for, if possible, to improve quality and yield. The present invention introduces expedients which equalize thermal exchange between the temperature control medium and different areas of a relatively large heat load such as a modern semiconductor wafer. Such improvements are particularly suitable for TDSF system but can be applicable to other thermal control systems as well.
It is known to vary the temperature of refrigerant gas within a thermal transfer fluid loop, as described in Cowans U.S. Pat. No. 6,775,996, issued Aug. 17, 2004. That system, however, is not a TDSF system, and the expedients described are not applicable to the problem of providing differential thermal energy exchange between different parts of a thermal load, particularly a semiconductor wafer.